Magnetic random access memory (MRAM) cells are often based on a magnetic tunnel junction (MTJ) cell comprising at least three basic layers: a “free” ferromagnetic layer, an insulating tunneling barrier, and a “pinned” ferromagnetic layer. In the free layer, magnetization moments are free to rotate under an external magnetic field, but the magnetic moments in the “pinned” layer are not. The pinned layer may comprise a ferromagnetic material and/or an anti-ferromagnetic material which “pins” the magnetic moments in the ferromagnetic layer. A thin insulation layer forms a tunneling barrier between the pinned and free magnetic layers.
In order to sense states in the MTJ configuration, a constant current can be applied through the cell. As the magneto-resistance varies according to the state stored in the cell, the voltage over the memory cell can be sensed. To write or change the state in the memory cell, an external magnetic field can be applied that is sufficient to completely switch the direction of the magnetic moments of the free magnetic layer.
A Tunneling Magneto-Resistance (TMR) effect is often utilized in conventional MTJ configurations. The TMR effect allows magnetic moments to switch directions in a magnetic layer in response to exposure to an external magnetic field. By utilizing the TMR effect, the magneto-resistance (MR) of an MTJ configuration may be altered. MR is a measure of the ease with which electrons may flow through the free layer, the tunneling barrier, and the pinned layer. A minimum MR occurs in an MTJ configuration when the magnetic moments in both magnetic layers have the same direction or are “parallel.” A maximum MR occurs when the magnetic moments of both magnetic layers are in opposite directions or are “anti-parallel.”
The most common structure for writing to MRAM cells comprises a set of orthogonal write lines, wherein one write line is parallel to the easy axis of the corresponding MRAM cell and the other write line is perpendicular to easy axis (or parallel to the hard axis). An electrical current is introduced to the write line to create the magnetic field necessary to change the direction of the free layer magnetization. However, the path of the resulting magnetic flux of the MRAM cell free layer or pinned layer does not form a closed loop. Consequently, the magnetic flux causes a shift in the corresponding B-H loop and asteroid curve. Increased surface roughness of one or more of the free layer, pinned layer, and tunneling barrier also causes a shift in the corresponding B-H loop and asteroid curve. The shift in the asteroid curve leads to an asymmetric writing threshold, or a switching threshold shift, resulting in bit errors and reducing performance and reliability.